536 VISUAL PERCEPTIONS. [BOOK in. 



vision may be said to be limited to the macula lutea, or even to 

 the fovea centralis ; by continual movements of the eye we are 

 constantly bringing any object which we wish to see in such a 

 position that its image falls on this region of the retina. 



The diminution of distinctness does not take place equally from 

 the centre to the circumference along all meridians. The outline 

 described by a line uniting the points where two spots cease to be 

 seen as two when moved along different radii from the centre, is a 

 very irregular figure. 



The sensations of colour are much more distinct in the centre 

 of the retina, than towards the circumference. If the visual axis 

 be fixed and a piece of coloured paper be moved towards the out- 

 side of the field of vision, the colour undergoes changes and is 

 eventually lost, red disappearing first, and blue last, the object re- 

 maining visible, though with very indistinct outlines, when its 

 colour can be no longer recognised. A purple colour becomes 

 blue, and a rose colour a bluish white. In fact, there seems to be 

 a certain amount of red-blindness in the peripheral parts of all 

 retinas. 



Modified Perceptions. 



Since our perception of external objects is based on the dis- 

 tinctness of the sensations which go to form the perception, it 

 might be expected that when an image of an object is formed on 

 the retina the sensory impulses would correspond to the retinal 

 image, the sensations correspond to the sensory impulses and the 

 perception correspond to the sensations, and that therefore the 

 mental condition resulting from our looking at any object or view 

 would correspond exactly to the retinal image. We find, however, 

 that this is not the case. The sensations and probably even the 

 simple sensory impulses produced by an image react upon each 

 other, and these reactions modify our perceptions, independently of 

 the physical conditions of the retinal image. There arise certain 

 discrepancies* between the retinal image and the perception, some 

 having their source in the retina, some in the brain, and others 

 being of such a nature, that it is difficult to say where the 

 irrelevancy is introduced. 



Irradiation. A white patch on a dark ground appears larger, 

 and a dark patch on a white ground smaller, than it really is. 

 This is especially so when the object is somewhat out of focus, and 



